Avalonia

The rocks of the main block of Avalonia as they relate to modern coasts and boundaries but in their relative positions as they were at the end of the Carboniferous, before Europe and North America separated again. The names are given in their French forms.

Avalonia is a former microcontinent, and the region of its Avalon terrane. It underlies south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States. Avalonia is named for the Avalon Peninsula in Newfoundland, which is one place it can be seen.

Avalonia developed as a volcanic arc on the northern margin of Gondwana. It eventually rifted off, becoming a drifting microcontinent. The Rheic Ocean formed behind it, and the Iapetus Ocean shrank in front. It collided with Baltica, then Laurentia, and finally with Gondwana, ending up in the interior of Pangea. When Pangea broke up, Avalonia's remains were divided by the rift which became the Atlantic Ocean.

Development

The early development of Avalonia is believed to have been in volcanic arcs near a subduction zone on the margin of Gondwana. Some material may have accreted from volcanic island arcs which formed further out in the ocean and later collided with Gondwana as a result of plate tectonic movements. The igneous activity had started by 730 million years ago and continued until around 570 million years ago, in the late Neoproterozoic.

In the early Cambrian, the supercontinent Pannotia broke up and Avalonia drifted off northwards from Gondwana. This independent movement of Avalonia started from a latitude of about 60° South. The eastern end of Avalonia collided with Baltica, a continental plate occupying the latitudes from about 30°S to 55°S, as Baltica slowly rotated anticlockwise towards it. This happened at the end of the Ordovician and during the early Silurian.

In the late Silurian and lower Devonian, the combined Baltica and Avalonia collided progressively, with Laurentia, beginning with the long extremity of Avalonia which is now attached to the USA and Canada. The result of this was the formation of Euramerica. At the completion of this stage, the site of Britain was at 30°S and Nova Scotia at about 45°S. This collision is represented by the Caledonian folding or in North America as an early phase in the Acadian orogeny.

In the Permian, the new continent and another terrane, Armorica which included Iberia, drifted in from Gondwana, trapping Avalonia between it and the continent so adding Iberia/Armorica to Euramerica. This was followed up by the arrival of Gondwana. The effects of these collisions are seen in Europe as the Variscan folding. In North America it shows as later phases of the Acadian orogeny. This was happening at around the Equator during the later Carboniferous, forming Pangaea with Avalonia near its centre but partially flooded by shallow sea.

In the Jurassic, Pangaea split into Laurasia and Gondwana, with Avalonia as part of Laurasia. In the Cretaceous, Laurasia broke up into North America and Eurasia with Avalonia split between them.

Iberia was later rotated away again as the African part of Gondwana brushed past. This last movement caused the Alpine orogeny including the raising of the Pyrenees during the Miocene and Pliocene. As a result of this, part of Avalonia is now to be found on each side of the Straits of Gibraltar.

This map shows the positions of the rocks of Avalonia remaining in Europe. The notes on it indicate the part which collided with Baltica in the upper Ordovician and that which collided with Laurentia in the Silurian. The parts of Avalonia now in Iberia and Morocco were carried there by rotation of Iberia during the subsequent collision with Gondwana followed by separation. These rocks are by no means all at the modern surface.

Consequences

In the modern world, we see Avalonia as forming the basic structure of the Ardennes of Belgium and north-eastern France, north Germany, north-western Poland, England, Wales, south-eastern Ireland, the south-western edge of the Iberian Peninsula, the Avalon Peninsula, much of Nova Scotia, southern New Brunswick and parts of New England.

The Avalonian part of Britain almost exactly coincides with England and Wales. Part of this which had formed an island in the Carboniferous, so affecting the disposition of coalfields, is known by names such as the 'London-Brabant Island'. Its bulk had an effect on the geological structure between the Ardennes and the English Midlands by influencing the subsequent crustal folding resulting from the Variscan collision.

In Canada, Avalonia comprises the Avalon Peninsula of southeast Newfoundland, portions of New Brunswick, Nova Scotia, Prince Edward Island. In the United States, Avalonia comprises northern coastal Maine, other sections of coastal New England, and sections of coastal North Carolina.

Avalonia underlies much of the eastern Appalachian Mountains, from Newfoundland to New England.

Maps

• [1] These maps show the relative positions of the terranes and continental plates at various stages in the Ordovician, Silurian and Devonian respectively. The names shown are in their German forms.

See also

• Iceland - a microcontinent
• Mistaken Point (Newfoundland and Labrador)

External links

• Article about Avalonia at Palaeos.Com (licensed under Attribution-NonCommercial 1.0 complete with pictures)
• The context of Avalonia's movements (not available without registration)
• Relative positions of Avalonia and Gondwana